I had a reference 5870 that used to pass Vantage at 1080/1300 with the stock cooler ( no mods, just raised Vgpu via software )... but with Dry Ice & LN2 it wouldn't get past 1300GPU & 1250 mem in any benchmark

Similarly, my newest ref. 5870 maxed out at 1055/1300 Vantage with the stock cooler, but did 1350/1240 on LN2.

I don't think pretesting on air/stock can be considered a good indication of how it might overclock at subzero temps.